1. Field of the Invention
The present invention relates to a method for controlling the operation of a pump and also to a system enabling this method to be applied.
With the Euro IV standard on exhaust emissions from heavy goods vehicles coming into effect in 2005, devices for pollution control of NOx (or nitrogen oxides) have had to be put in place.
The system used by most heavy goods vehicle manufacturers for reducing NOx emissions to the required value generally consists in carrying out a selective catalytic reaction with reducing agents such as urea (“Urea SCR” or selective catalytic reduction using ammonia generated in situ in the exhaust gases by decomposition of urea).
In order to do this it is necessary to equip the vehicles with a tank containing a urea solution, a device for metering the amount of urea to be injected into the exhaust line and a device for supplying urea solution to the device for metering the amount of urea to be injected. Generally, the supply device comprises a pump driven by a motor.
Preferably, the operation of this pump is controlled by means of a controller which can act on the operating pressure of the pump, the rotation direction of the pump (either to supply urea, or to purge the conduits), the starting and/or stopping of the pump and/or to carry out a diagnosis of the operating state of the pump.
2. Description of Related Art
In the prior art, several systems have been proposed for controlling the operation of a pump.
Thus, U.S. Pat. No. 5,670,852 describes a device for controlling the speed of an electric motor driving a pump that acts only on the speed of the drive motor, independently of the pressure at the inlet and outlet of the pump. The outlet pressure of the pump is consequently not controlled. The control device regulates the speed of the motor from two input data: a motor speed measurement signal supplied by commutation sensors and a motor speed setpoint signal. The latter is either a voltage or a frequency of a square wave. The control device according to U.S. Pat. No. '852 makes it possible, using a single signal (44), to control all the operating modes of the pump (forward drive, reverse drive). However, it is complex and requires the use of a comparator, a phase-locked (or frequency-locked) loop, commutation comparators, etc. Moreover, it does not make it possible to carry out a diagnosis of the pump operation and, in particular, to identify a risk of explosion in the case where a plug is formed at the pump outlet or a risk of pollution if there is a leak in the supply circuit, since the pressure at the outlet of the pump is not controlled.
Application US 2002/0043253 discloses a system for regulating a pump that makes it possible to solve this problem by directly controlling the pressure at the outlet of the pump. This device comprises a controller (36) which receives a pressure setpoint value from an electronic control module (38), compares this value with that measured by a pressure sensor (30) to create an error signal and generate a modulated (PWM or Pulse Width Modulated) voltage which directly controls the rotational speed of the electric motor. U.S. Pat. No. 5,237,975 describes a similar system.
The system described in these documents is specific to fuel systems, where the electric motor is either running or at rest, but has only one direction of rotation. Especially for the urea pumps mentioned above, it is advantageous to also provide a reverse direction of rotation in order to be able to carry out purge cycles. Moreover, the urea circuits may get blocked up (especially after freezing of the solution) and/or have leaks, in which case the pump is advantageously stopped. However, the system described in the aforementioned documents does not make provision for carrying out a diagnosis on the operation of the pump.
Finally, in the case of systems for the injection of liquid (such as urea) that incorporate a specific control system for the pump, it is necessary to provide an interface between this system and a more central control module (for example that of the vehicle or of a complete injection system) that makes it possible, with the fewest possible electrical connections, to exchange all the information needed for the correct operation of the pump.